After a tedious investigation, it appears that the problem lies in the underlying iframe: setting the iframe style to "height: 100%" doesn't work in HTML in general. You can set the height in pixels, but it's up to you to figure out how many pixels high it should be to fit properly. There's a related but different problem for the TabPanel: 100% seems not to respect other widgets that it's packed with, so it ends up too large, resulting in a vertical scroll bar where you don't want it. Incidentally, "width=100%" works fine in both cases.

You also need to call both these functions initially as part of the setup code. This seems to be an unfortunate case where you need to know more than GWT's surface: you need to understand the underlying HTML implementation to solve the problem.